Described below are a method and an apparatus for allocating an identity to user equipment (UE) during a random access procedure in a radio communications system.
For UEs 10 to transmit and receive messages in a radio communications system 1000, a UE 10 has to gain access to an access node 100, in order to be able to set up a connection with access node 100 and consequently access the internet, the PSTN (Public Switched Telephony Network) or other radio communications systems.
To gain access to access node 100, UE 10 has to execute a random access procedure. During the random access procedure, a UE 10 uses a RACH (random access channel) to transmit messages requesting radio resources.
In a random access procedure four messages are exchanged between UE 10 and an access node (AN) 100, as indicated in the UTRAN LTE (UMTS Terrestrial Radio Access Network Long Term Evolution) standard 36.300 v080. This can be seen in FIG. 1. In step 1, UE 10 transmits a random access preamble message to access node 100. In step 2, the access node 100 transmits a random access response message. In step 3, a scheduled transmission message is transmitted from UE 10 to access node 100, and in step 4 a contention resolution message is transmitted by access node 100.
During step 1, UE 10 transmits over the RACH a signature identifying itself to AN 100. This signature can be one out of 64 possible signatures.
During step 2, AN 100 transmits an assigned C-RTNI (Cell-Radio Network Temporary Identifier) identity for every signature that it has received in a certain time instant. This identity also includes other parameters, for example, a TA (Timing Advance) and a UL-RA (Uplink Resource Allocation) which are used by UE 10 for the transmission in step 3. The random access response message of step 2 is transmitted over a downlink shared channel (DLSCH) using fast retransmissions instead of HARQ (Hybrid Automatic Repeat Request). The identity used on the DLSCH is a RA-RNTI (Random Access-Radio Network Temporary Identifier) which is unique for a specific random access occasion and addresses all UEs 10 using the specific occasion. In this way, all UEs 10 know their corresponding RA-RNTI in advance and upon identifying it on the DLSCH, a UE 10 can then proceed with reading the received random access response message.
As a plurality of UEs 10 can use the RACH during the same time instant or occasion up to 64 different signatures can be received by AN 100 in a single time instant or occasion. This will result in a message transmitted in step 2 carrying up to 64 C-RNTI identities and parameters. Considering that the C-RTNI has a size of 16 bits, such a message can have a maximum size of 1024 bits (64*16 bits) for the C-RNTI payload. FIG. 2 provides a table showing the allocation of bits that include the C-RNTI payload.
FIG. 3, illustrates a table showing the allocation of bits of the C-RNTI payload, according to one solution, wherein the size of the C-RNTI payload is reduced. According to this solution, the C-RTNI payload is shortened for each signature and has a size of 8 bits. Considering that the C-RTNI has a size of 8 bits, such a message can have a maximum size of 512 bits (64*8 bits) for the C-RNTI payload. The final C-RTNI for each specific signature is obtained by combining the shortened C-RTNI with a base C-RTNI (C-RTNIO in FIG. 3).
Both schemes described herein above it is also possible to split the message transmitted in step 2 in a number of messages transmitted within a certain time window or frame using the same RA-RTNI. This allows for a reduction in the size of the C-RTNI payload and consequently a reduction in the overall size of the message transmitted in step 2.
However, this has the disadvantage that UEs 10 awaiting such a message will have to read all such messages addressed by the RA-RNTI until they can find their respective transmitted signature. This will increase the waiting time that a UE 10 has to wait before it can then proceed with the random access procedure. Such a waiting delay or period can cause a UE 10 to loose the opportunity to gain access as during this waiting period another UE 10 can gain access.
In addition, further drawbacks are that in the event that the payload increases radio resources need to be allocated. This wastage of radio resources reduces the efficiency of the radio communications system as less radio resources are available for other uses. Furthermore, when transmitting messages without the use of HARQ, in order to ensure that messages can be received correctly at the edges of cells, higher order modulation is required. This increases the complexity of the access nodes and consequently the costs. In addition, the larger the payload size of the C-RTNI, increases the risk of errors being present and increases the probability of failed reception and consequently increases the number of re-transmissions to be performed in order for the messages to be correctly received. Such re-transmissions will cause a further the completion of the random access procedure.
A need therefore exists for a technique that overcomes the above mentioned drawbacks and ensures that a random access procedure is efficiently and that a user equipment can receive its identity in as short a time as possible completed. With the method described herein, the above mentioned problems are resolved. The proposed technique provides for an efficient and fast exchange of messages during a random access procedure.